# Calculation of Rin,out of audio filter

Discussion in 'Homework Help' started by Fovakis, Nov 7, 2013.

May 14, 2013
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2. ### MikeML AAC Fanatic!

Oct 2, 2009
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The input impedance is hard to determine, likely > 20K. However, the Sallen-Key filter section will not work as you expected unless it is driven from a voltage source. It needs to be driven from a low impedance (buffer?), so I would say that the input impedance is zero, and you will have to design the upstream circuit to drive a low impedance, or add the requred buffer.

The output impedance is a voltage source (~0Ω) in series with the output blocking capacitor. However, when I say that, it doesn't mean that you can connect a 4Ω speaker there. An opamp can only drive a small, finite current (<20ma?) into an external load.

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3. ### The Electrician AAC Fanatic!

Oct 9, 2007
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The input impedance varies with frequency, and looks like this (assuming I read all your component values correctly):

• ###### Zin.png
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4. ### Fovakis Thread Starter New Member

May 14, 2013
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So the frequency bandwidth is DC to 17khz (audio)! Before this filter there is a double balanced ring diode mixer that downconvert a signal of 30MHz to a baseband DC to 17khz. The mixer's output (ideally) is 50ohm.

RF front....---Mixer-----Active Filter--Demodulator (DSP). I want to calculate the total gain of the stages of receiver but in the filter i don't know the Zinput and Zoutput to convert the voltage gain=20log(Vout/Vin) to power gain. I need to know what is the Zout and Zin. After the Active filter there is a DSP kit i think it's Zinput is about 10k maybe not sure...

5. ### Fovakis Thread Starter New Member

May 14, 2013
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Hi ! please check the answear that i already gave to the other person. how you did that? is a Zinput to Frequency diagram?

6. ### The Electrician AAC Fanatic!

Oct 9, 2007
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Impedance in ohms on the vertical axis and frequency in Hz on the horizontal axis.

I did it by setting up the network equations for the circuit and solving for Zin.

Once you leave the mixer, you are no longer dealing with signals in a 50 ohm impedance environment. Normally one doesn't calculate power gain in an audio frequency environment.

The output impedance of the active filter is essentially zero ohms. You need to use the impedance driven by the output of the filter to determine how much power you are delivering to that load.

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7. ### Fovakis Thread Starter New Member

May 14, 2013
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can you help me setting up the network equations? how i do that?

8. ### The Electrician AAC Fanatic!

Oct 9, 2007
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This is the homework help forum. The way it works is that you have to show some effort to solve the problem yourself.

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9. ### Fovakis Thread Starter New Member

May 14, 2013
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Sure, but i don't know how to START to solve the problem. You just gave me a diagram.. i cannot get helped with that.

10. ### The Electrician AAC Fanatic!

Oct 9, 2007
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Number the nodes in the filter and set up a KCL equation for each node.

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11. ### Fovakis Thread Starter New Member

May 14, 2013
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Οh....yes you have right Electrician i will work on that thaks